The purpose of this study was to determine the influence of age on the ability to adjust nutrient oxidation to changes in diet composition. We studied 6 young (YM; age=25+1 yrs) and 5 older (OM; age=63+3 yrs) men after 4 days of consuming each of 3 isocaloric diets containing a constant %protein but varying in %carbohydrate and %fat (order randomized): mixed diet (M;15/30/55), high fat diet (HF;15/60/25), and high carbohydrate (HC; 15/15/70). Each subject underwent 24-hr whole-room calorimetry on day 4 of each diet to determine 24-hr nutrient oxidation rates. Body mass, %fat, and fat-free mass were similar in the two groups. 24-hr EE and energy balance did not differ across diets or between groups; 24-hr EE was ~7% lower (ns) in the OM. Nutrient oxidation was similar in Y vs. O during M. Protein oxidation was similar across diets, but higher (P<0.05) in OM. Fat oxidation increased to 58.4+14.9 vs. 51.9+13.0% of 24-hr EE (ns) in the OM vs. YM, respectively, during HF and decreased to 25.4+9.7 vs. 20.2+14.3% (ns) during HC (diet effect, P<0.05). Carbohydrate oxidation decreased to 19.5+10.6 vs 29.9+12.6% (ns) during HF and increased to 53.6+12.3 vs. 64.7+14.3% (ns) in the OM vs. YM, respectively during HC (diet effect, P<0.05). Taken together, these results suggest that OM maintain the ability to adjust nutrient oxidation to changes in diet composition.